1. Field of the Invention
The present invention relates to a II-VI group compound semiconductor light-emitting device, used chiefly for a display apparatus, and further relates to its manufacturing method.
2. Description of the Prior Art
Recently, there has been developed a display light source of, for example, a wall type TV, comprising a blue luminescence color light-emitting diode (hereinafter, referred to as an LED). These light sources are expected to have good brightness and efficiency is expected. ZnSe and ZnS or ZnSSe of the II-VI group compound semiconductor have band gaps of 2.7 to 3.7 eV at room temperature, and their band structures are categorized into a direct transition type. Therefore, these II-VI group compound semiconductors are expected to be used as blue luminescence color LED semiconductor materials.
In general, in order to realize a blue luminescence color LED having high brightness and high efficiency, it is necessary to dope impurities into these materials to carry out the conduction type control so that a P-N junction structure can be accomplished. However, it is also known that it is difficult to obtain a low-resistivity p-type conduction from these ZnSe, ZnS or their mixed crystal ZnSSe.
Therefore, in order to obtain a low resistivity p-type conduction, numerous trials have been carried out in seeking a new acceptor impurity. An already reported result is that a p-type conduction can be realized by doping N or Li in ZnSe as disclosed in the Journal of Applied Physics 59 (1986) 2256, reported by J. Nishizawa, and also in the Journal of Crystal Growth 93 (1988) 692, reported by A. Ohki et al., or the Applied Physics Letters 52 (1988) 57, reported by Y. Yasuda et al.
FIG. 16 shows a construction of a p-n junction type blue luminescence color LED obtained by doping N impurities in ZnSe, as a conventional II-VI group compound semiconductor light-emitting device. A reference numeral 201 denotes a Si doped n-type GaAs substrate, and a reference numeral 202 denotes an Cl doped n-type ZnSe epitaxial layer. A reference numeral 203 denotes a N doped p-type ZnSe epitaxial layer. Reference numerals 204 and 205 denote Au and AuSn electrodes, respectively.
If a forward bias voltage is applied to this p-n junction structure, electrons as minority carriers are injected from the n-type ZnSe epitaxial layer 202 to the p-type ZnSe epitaxial layer 203 so as to obtain light-emission.
To the contrary, there was no report of the realization of low resistivity p-type conduction based on the doping of these impurities in the light-emitting device when adopting ZnS as the II-VI group compound semiconductor. That is, a p-n junction type blue luminescence color LED has not yet been obtained.
On the other hand, a ZnSe light-emitting device used as a conventional II-IV group compound semiconductor has the following problems.
Even if N or Li is doped as an impurity into the conventional ZnSe light-emitting device, it is not possible to obtain a practically sufficient hole concentration more than a carrier concentration p.about.10.sup.16 cm.sup.-3. And, in the case where highly concentrated impurities of N or Li are forcibly doped into the ZnSe, red luminescence is chiefly generated from the deep acceptor level formed in the band gap as shown in FIG. 17. Namely, there was a problem in that a blue luminescent color could not be obtained in the conventional ZnSe light-emitting device.
Furthermore, a structure of the conventional ZnSe light-emitting device is a homo junction/hetero structure consisting of a p-n junction of a p-type ZnSe layer and an n-type ZnSe layer. That is, the structure of the conventional ZnSe light-emitting device is not a double hetero structure laminating more than three layers of semiconductor layers, each having different band gap energy. Therefore, it is not possible to confine highly concentrated electrons and photons in an active region causing light-emissions. Also, there were other problems in that light-emission efficiency could not be improved and stimulated emission (i.e. laser oscillation) could not be obtained in the conventional structure.
Moreover, in a light-emitting device adopting ZnS as II-VI group compound semiconductor, a low resistant p-type conduction cannot be obtained even if impurities are doped. Consequently, the realization of blue luminescence color is difficult in the conventional light-emitting device.